A switch mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Controllers associated with the power converter manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage) of the power converter and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). Additionally, as the needs for systems such as a microprocessor powered by the power converter dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain the output characteristic at a desired value.
In combination with a controller, a driver is often employed to provide a drive signal to the switches of the power converter as a function of a signal from the controller. Assuming without limitation that the switches of the power converter are metal-oxide semiconductor field-effect transistors (“MOSFETs”), the driver is referred to as a gate driver and provides a gate drive signal (i.e., a drive signal) to a gate terminal (i.e., a control terminal) of the MOSFET to control an operation thereof. Providing a gate drive signal with a limited control voltage range (or “gate voltage limit”) for a MOSFET is of particular interest in the design and implementation of power converters. In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as five volts, to a lower, regulated, output voltage, such as 2.5 volts, to power a load.
As discussed above, power converters are frequently employed to power loads having tight regulation characteristics such as a microprocessor with, for instance, five volts provided from a source of electrical power (e.g., a voltage source). To provide the voltage conversion and regulation functions, the power converters include active switches such as the MOSFETs that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of ten megahertz. To maintain high power conversion efficiency and low cost, the MOSFETs employed for the switches in the power converters are generally configured with fine line structures and thin gate oxides. The aforementioned structures that form the MOSFETs, however, present new design challenges associated with maintaining voltage limits of the control signals such as a gate voltage adapted to control the conduction periods of the switches.
For instance, recently designed MOSFETs for the power converters can reliably sustain control signals of no more than, for instance, about 2.5 volts from the gate terminal to the source terminal, whereas MOSFETs of earlier designs were able to sustain control signals of 20 volts or more. Additionally, the power converters often employ a P-channel MOSFET as a main power switch therein. Inasmuch as the P-channel MOSFET is generally coupled to the input voltage source (e.g., nominal five volts) of the power converter, the gate voltage is desirably controlled to a value of the input voltage (again, five volts) supplied by the input voltage source to transition the switch to a non-conducting state. Conversely, the P-channel MOSFET is enabled to conduct at a gate voltage equal to the input voltage of five volts minus 2.5 volts, which represents about the maximum sustainable voltage from the gate terminal to the source terminal of the switch (also referred to as a “gate-to-source voltage limit” or a “gate voltage limit” of the switch).
It is common to employ a driver for a P-channel MOSFET that includes a series-coupled, totem-pole arrangement of a P-channel and N-channel MOSFET with coupled gate terminals. In the environment of a power converter, the totem pole driver (as the driver is customarily designated) for the P-channel MOSFET is coupled to a positive source of electrical power for the power converter and to the controller of the power converter. The drive signal for the gate of a P-channel MOSFET is generated from a junction coupling the drain terminals of the P-channel and N-channel MOSFETs of the totem pole driver. When a signal from the controller to the totem pole driver is high, the drive signal is essentially grounded. When the signal from the controller to the totem pole driver is low, the drive signal is substantially equal to the input voltage of the power converter. In effect, the drive signal from the totem pole driver exhibits voltages over the entire voltage range of the source of electrical power for the power converter, which can exceed the allowable voltage range of the gate of the driven switch, which may be limited to 2.5 volts from its source terminal, as described above. Alternatively, the driver may be described as providing a drive signal referenced to ground when its output is low, and a drive signal referenced to the input voltage when its output is high, thereby exceeding the voltage range of the gate of the driven switch.
Thus, when providing a drive signal to a P-channel MOSFET (or any switch for that matter, such as an N-channel MOSFET) having a gate voltage limit of 2.5 volts, and in the environment of a power converter for example, having a nominal input voltage of five volts, the extended voltage range present on the gate terminal of the switch may break down the integrity of the thin gate oxide of the switch. In other words, when the input voltage to the power converter which is translated into the drive signal to the switch under certain conditions as described above exceeds the gate voltage limit thereof, the switch may be damaged and fail. Thus, the totem pole driver and other presently available drivers are typically not practical without circuit modifications for applications wherein the switch to be driven exhibits a smaller gate voltage limit than the input voltage of the power converter from the gate terminal to the source terminal thereof.
Another level of complexity arises when the switch to be driven and the driver are referenced to different voltages. In the environment of the power converter described above, circuitry that embodies the driver may be coupled to ground and referenced to a ground potential and the switch to be driven may be referenced to, for instance, the input voltage of the power converter. As a result, the driver is referenced to a ground potential and the switch is referenced to an input voltage such as an unregulated five volt input voltage. Thus, in conjunction with the control voltage limit, the driver should be adapted to drive a switch referenced to another voltage level as described above and adaptively perform the necessary voltage translation to the another voltage level to provide the drive signal.
There have been attempts to meet the voltage limitations of a switch as described above. Generally, an intermediate bus voltage is established for a driver and by-passed to a reference potential such as ground with substantial added capacitance. Additionally, a level-shifting capacitor may be added in conjunction with the driver to provide a voltage translation function therefor. The addition of a capacitor to a controller with the driver (which is generally formed as an integrated circuit) usually requires the addition of two conductive “pads” to accommodate an external capacitor interconnection, which consumes substantial die area, or the substantial space required for an integrated capacitor. Thus, drivers for applications employing switches with a control voltage limit (i.e., a control voltage substantially less than an input voltage of the power converter) are often allocated space that is not easily integrable with the rest of the integrated circuit. Consequently, controllers with such drivers add measurable cost to a design of a power converter beyond the voltage sourcing functionality thereof.
Accordingly, what is needed in the art is a driver, and a method of driving a switch, that takes into account a control voltage limit associated with a switch (e.g., a gate voltage limit for a MOSFET) referenced to a voltage level different from the driver, that overcomes the deficiencies in the prior art.